Golden Gate Bridge wind tests underway for seismic work

The Golden Gate Bridge is in the latter stages of a seismic retrofitting project that began in 1998. (Alan Dep/Marin IJ archives)

A miniature mid-section of the Golden Gate Bridge is now being buffeted by intense winds in a Canadian lab to make sure the real thing won’t get damaged in case of an intense storm.

Bridge officials are working to seismically strengthen the center portion of the span and wind testing is a key element of the work. Engineers are always concerned that any modification to a bridge, however slight, could affect how it performs in high winds.

Seismic work on the center suspension portion of the span will involve platforms and the replacement of a top lateral bracing system. That will require installation of temporary bracing supports to maintain structural integrity and load carrying capacity of the span.

Bridge engineers want to conduct wind tests to make sure the temporary pieces and platforms can survive the sometimes howling winds that can slam into the bridge deck that sits 220 feet above the water.

Earlier this month bridge staff traveled to Ontario, Canada, to watch winds up to 100 mph blow against the model. Smoke was used at some points to get a visual on wind speeds.

The bridge board approved a $191,000 contract to Rowan Williams Davies and Irwin Inc. for the work in Guelph, Ontario.

“The work is ongoing,” said Priya Clemens, bridge district spokeswoman. “They will continue to test to see what works in high-wind conditions.”

Seismic work on the northern approach, southern approach and the north anchorage house — the box-like structure that holds all the cables coming off the span that tie down the bridge — has been completed.

In terms of seismic work, the center suspension segment was saved for last because it is least susceptible to collapse in a major quake, bridge officials said. The final phase of the seismic effort is pegged between $450 million and $500 million.

“This is an opportunity to better understand how the bridge would handle potentially big wind events with additional platforms,” said Denis Mulligan, bridge general manager, of the wind testing. “Long span suspension bridges, like ours, can be susceptible to damage in high winds, so we’re making sure the bridge, workers, and drivers are safe as we complete our seismic retrofit.”

Wind can wreak havoc. Most often cited is the colossal failure of the original Tacoma Narrows Bridge in Washington state, nicknamed “Galloping Gertie” before it collapsed in 1940 because of winds, just months after it opened.

It has been a long journey for the larger Golden Gate Bridge seismic project, which began in 1998. Initially the work was to have cost roughly $300 million but has since increased to more than $900 million as prices have gone up over time and bridge officials were forced to further gird the bridge after the 9/11 attacks.

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Presently the bridge is considered safe, with officials saying the north and south ends could sustain an 8.3-scale quake. The suspension section could withstand a large earthquake, bigger than a 7.0, but there would be extensive damage and the bridge would likely have to be closed for a long period. About 110,000 people use the span daily.

The 80-year-old bridge was given a “good” rating by federal inspectors in terms of condition last year. Only an “excellent” is a better designation and typically given to new spans.